1. Field of the Invention
The present invention relates to an image forming apparatus such as an electronic copying apparatus, which has a cooling fan for preventing an excessive temperature rise within the apparatus.
2. Description of Related Art
Most electronic copying apparatuses have many components (e.g., an exposure lamp) which consume much electric power. The electric power of the apparatus consumes during operation amounts to about 1.5 KW. Part of the power consumed changes to heat, which raises the temperature within the apparatus. If the temperature is excessively high, it will damage to some of the components of the apparatus. To prevent such damages, a cooling fan is incorporated in the apparatus and is rotated at speed as high as approximately 2400 rpm, thereby maintaining the temperature below a specific value. The noise, which the fan makes while rotating, is great, annoying those working near the copying apparatus.
The period of time the copying apparatus remains in the standby condition is much longer than the period the apparatus is performing a copying operation. The copying apparatus is operated to copy originals from time to time, each time for a relatively short period. Thus, in the prior art, the cooling fan is rotated at a lower speed while the apparatus is in the standby condition, whereby a requirement for reducing the noise can be realized to some degree.
Although the copying apparatus makes less noise while set in the standby condition, it still makes noise. Even if the cooling fan is rotated during the standby period at the speed half that value during the copying period, the noise cannot be decreased as much as is desired by the people working near the apparatus. To decrease the noise as much as desired, various attempts have been made. More specifically, the cooling fan is rotated at a still lower speed, or it is stopped at all. Nonetheless, no satisfactory results have been attained. This is because the fan must have one cooling efficiency right after a copying operation and another cooling efficiency some time after the copying operation since, due to the heat capacity and time constant of the apparatus, the temperature and temperature distribution which the apparatus has immediately after a copying operation are different from those which the apparatus has some time after a copying operation. This is also because the efficiencies (i.e., the rotational speeds) required of the fan for cooling the heat-generating components are different because of the heat capacities of these components and the positions thereof with respect to the fan.